Is necessary because if any airlines or airlines of purchase requirements, prices will drop and turn to other airlines instead. Therefore, if a little bit moldy khaplian the purchase requirement.
Is necessary because if any one airline, airlines raised prices and reduced demand, it will turn to the services of other airlines instead. So if mold change little demand, it will change.
Is necessary because if any one airline airline price demand will be reduced and turned to the other airlines. So if RA change a little demand will change.